Set the error reporting level. The parameter is either an integer
representing a bit field, or named constants. The error_reporting
levels and constants are described in
Predefined Constants,
and in php.ini. To set at runtime, use the
error_reporting() function. See also the
display_errors directive.

PHP 5.3 or later, the default value
is E_ALL &
~E_NOTICE &
~E_STRICT &
~E_DEPRECATED. This setting does not
show E_NOTICE, E_STRICT
and E_DEPRECATED level errors. You may want
to show them during development.
Prior to PHP 5.3.0, the default value
is E_ALL &
~E_NOTICE &
~E_STRICT.
In PHP 4 the default value is E_ALL
& ~E_NOTICE.

Note:

Enabling E_NOTICE during development has
some benefits. For debugging purposes: NOTICE messages will warn you
about possible bugs in your code. For example, use of unassigned values
is warned. It is extremely useful to find typos and
to save time for debugging. NOTICE messages will warn you about bad style.
For example, $arr[item] is better to be written as
$arr['item'] since PHP tries to treat
"item" as constant. If it is not a constant, PHP assumes
it is a string index for the array.

Note:

In PHP 5 a new error level E_STRICT is available.
Prior to PHP 5.4.0 E_STRICT was not included within
E_ALL, so you would have to explicitly enable this kind of
error level in PHP < 5.4.0. Enabling E_STRICT during development
has some benefits. STRICT messages provide suggestions that can help
ensure the best interoperability and forward compatibility of your code.
These messages may include things such as calling non-static methods
statically, defining properties in a compatible class definition while
defined in a used trait, and prior to PHP 5.3 some deprecated features
would issue E_STRICT errors such as assigning
objects by reference upon instantiation.

Note:
PHP Constants outside of PHP

Using PHP Constants outside of PHP, like in httpd.conf,
will have no useful meaning so in such cases the integer values
are required. And since error levels will be added over time, the maximum
value (for E_ALL) will likely change. So in place of
E_ALL consider using a larger value to cover all bit
fields from now and well into the future, a numeric value like
2147483647 (includes all errors, not just
E_ALL).

Set the maximum length of log_errors in bytes. In
error_log information about
the source is added. The default is 1024 and 0 allows to not apply
any maximum length at all.
This length is applied to logged errors, displayed errors and also to
$php_errormsg.

When an integer is used, the
value is measured in bytes. Shorthand notation, as described
in this FAQ, may also be used.

If this parameter is set to On (the default), this parameter will show a
report of memory leaks detected by the Zend memory manager. This report
will be send to stderr on Posix platforms. On Windows, it will be send
to the debugger using OutputDebugString(), and can be viewed with tools
like » DbgView.
This parameter only has effect in a debug build, and if
error_reporting includes E_WARNING in the allowed
list.

Turn off HTML tags in error messages. The new format for HTML errors
produces clickable messages that direct the user to a page describing
the error or function in causing the error. These references are
affected by
docref_root and
docref_ext.

The new error format contains a reference to a page describing the error or
function causing the error. In case of manual pages you can download the
manual in your language and set this ini directive to the URL of your local
copy. If your local copy of the manual can be reached by "/manual/"
you can simply use docref_root=/manual/. Additional you have
to set docref_ext to match the fileextensions of your copy
docref_ext=.html. It is possible to use external
references. For example you can use
docref_root=http://manual/en/ or
docref_root="http://landonize.it/?how=url&theme=classic&filter=Landon
&url=http%3A%2F%2Fwww.php.net%2F"

Most of the time you want the docref_root value to end with a slash "/".
But see the second example above which does not have nor need it.

Note:

This is a feature to support your development since it makes it easy to
lookup a function description. However it should never be used on
production systems (e.g. systems connected to the internet).

Name of the file where script errors should be logged. The file should
be writable by the web server's user. If the
special value syslog is used, the errors
are sent to the system logger instead. On Unix, this means
syslog(3) and on Windows NT it means the event log. The
system logger is not supported on Windows 95. See also:
syslog().
If this directive is not set, errors are sent to the SAPI error logger.
For example, it is an error log in Apache or stderr
in CLI.
See also error_log().

You should also consider setting error_reporting = -1 in your php.ini and display_errors = On if you are in development mode to see all fatal/parse errors or set error_log to your desired file to log errors instead of display_errors in production (this requires log_errors to be turned on).

There does not appear to be a way to set a tag / ident / program for log entries in the ini file when using error_log=syslog. When I test locally, "apache2" is used. However, calling openlog() with an ident parameter early in your script (or using an auto_prepend_file) will make PHP use that value for all subsequent log entries. closelog() will restore the original tag.

This can be done for setting facility as well, although the original value does not seem to be restored by closelog().

When using PHP with Apache mod_fcgid and "log_errors = On", PHP errors get logged into Apache ErrorLog file with severity "warn". No matter what severity the PHP error itself has, the severity in the Apache log is "warn".To log PHP errors in the Apache log, use:LogLevel warn(or debug, info, notice)